As a type of power transmission device to be interposed between the front wheels and the rear wheels of a four-wheel drive vehicle, it is known to directly connect one of the front and rear wheel sets with the engine, and to transmit a drive torque from this axle (primary axle) to the other axle (secondary axle) via a viscous fluid coupling which responds to a relative rotational speed therebetween. Such a viscous fluid coupling is provided with the property to vary the transmission torque depending on the difference in the rotational speed between the primary axle and the secondary axle, and the primary and secondary axles are substantially coupled with each other when the difference in the rotational speed between the primary and secondary axles has exceeded a certain limit. In regard to this point, the mechanical strengths of the primary and secondary axle members against torque are required to be substantially equal to each other. 0n the other hand, when the difference in the rotational speed between the primary and secondary axles is extremely small, the transmission torque to the secondary axle is substantially equal to zero, and the burden of load on the secondary axle in such a case is extremely light. In view of such a recognition, it has been proposed in Japanese patent laid-open publication No. 63-49526 to reduce the load on the secondary driven axle members by defining an upper limit of torque transmission to the secondary driven axle, and to thereby reduce the overall weight of the drive system. In the following disclosure, the words "axle member" is intended to mean not only the shafts for front or rear wheels but also all the members situated in the path of torque transmission from the primary driven axle to the secondary driven axle.
It sometimes occurs that a vehicle has to run for an extended period of time using front and rear wheels having different diameters for instance when an emergency spare tire is used or when anti-slip devices such as chains are applied to the wheels in case of snow. In such a case, a difference in rotational speed between the front and rear wheels is continually present, and the secondary axle may receive a drive torque that is larger than actually required even when no torque transmission is required between the primary and secondary axles. In other words, even though only the transmission of torque between the front and rear axles is required to be considered as a transient process under normal running condition, a drive torque may be continually transmitted to the secondary axle when the diameters of the tires are not identical. This is not desirable because the clutch would be placed under a high load condition over an extended period of time, and/or the slipping of the tires may be induced.
It is generally known that a metallic member may rupture or break even when the stress applied thereto is less than its rupture stress if it is repeatedly applied to the member, and metal fatigue is thereby induced. In particular, when a vehicle is running at high speed, the number of the repeated applications of stress for a given time period increases accordingly, and the effective limit stress of the power transmission members may be lowered because of the possibility of metal fatigue. Therefore, in order for the vehicle to withstand the continual running of the vehicle equipped with tires having uneven diameters, the safety factor in the design process has to be increased. Therefore, simply defining the upper limit of torque transmission to the secondary axle alone, as was the case with the aforementioned prior art, is not sufficient to achieve the reduction in weight in practical sense.